
674 P.2d 617 (1984)
66 Or.App. 457
Inez VAN HORN, Petitioner,
v.
JERRY JERZEL, INC., Royal Globe Insurance, Charles F. Berg, Inc., Giesy, Greer & Gunn, Respondents.
Nos. 80-02851 and 80-05095; A28060.
Court of Appeals of Oregon.
Argued and Submitted November 21, 1983.
Decided January 11, 1984.
*618 James P. Cronan, Milwaukie, argued the cause for petitioner. With him on the brief was Roger N. Rook, Milwaukie.
Jerald Keene, Portland, argued the cause for respondents Royal Globe Ins. and Jerry Jerzel, Inc. With him on the brief was Griffith, Bittner, Abbott & Roberts, Portland.
Deborah S. MacMillan, Portland, argued the cause for respondents Charles F. Berg, Inc., and Giesy, Greer & Gunn. With her on the brief were Frank A. Moscato and Moscato & Meyers, Portland.
Before RICHARDSON, P.J., and VAN HOOMISSEN and NEWMAN, JJ.
VAN HOOMISSEN, Judge.
Petitioner appeals an order of the Workers' Compensation Board denying her aggravation claim against respondent Jerry Jerzel, Inc. (Jerzel) and, as to respondent Charles F. Berg, Inc. (Berg), making an inadequate penalty and attorney fee award based on Berg's failure to pay interim compensation. She alleges that her worsened back condition was causally related to her compensable industrial injury and that the Board's award of a ten percent penalty and $100 attorney fee was insufficient. We review de novo and affirm.
Petitioner, aged 64, was injured in February, 1978, while employed by Jerzel's. Her back strain claim was accepted, and she received compensation for time loss, ORS 656.210, but not for permanent disability. In April, 1978, she began working for Berg. She alleges that she sustained an injury at Berg's in December, 1979. She then filed a new injury claim against Berg and an aggravation claim against Jerzel. Both carriers denied responsibility, and the referee affirmed those denials. The Board affirmed the denials. However, it found that claimant was improperly denied interim disability payments, which it awarded, plus penalties and attorney fees. ORS 656.262(9).
*619 In order to establish her aggravation claim, claimant had the burden of proving that her condition had worsened and that the worsening was causally related to her industrial injury. Anderson v. West Union Village Square, 43 Or. App. 295, 297, 602 P.2d 1092 (1979), modified 44 Or. App. 685, 607 P.2d 196 (1980). Claimant places primary reliance on the medical reports of Dr. Weinman, who stated that he suspects claimant is suffering from a herniated disc.[1] He points to little objective evidence of that condition, however. He relies instead on claimant's subjective evaluation of her condition.
Evidence of a worsening of a lumbosacral spine condition must be supported by expert medical evidence. Jacobson v. SAIF, 36 Or. App. 789, 792, 585 P.2d 1146, rev. den. 284 Or. 521 (1978). In Oakley v. SAIF, 63 Or. App. 433, 436, 664 P.2d 431 (1983), we noted:
"An aggravation claim based solely on a claimant's statement that his condition has deteriorated is statutorily insufficient. Likewise, a medical report which only sets forth claimant's statement that his condition has deteriorated is insufficient. At the very least, an expression of opinion by the doctor that he believes the claimant or that he finds objective evidence from a medical standpoint to substantiate claimant's history, is necessary. Larson v. Compensation Department, 251 Or 478, 482, 445 P2d 486 (1968); Collins v. States Veneer, Inc., 14 Or App 114, 119, 512 P2d 1006 (1973)."
Dr. Post, who examined claimant's deposition testimony and medical records, characterized Dr. Weinman's diagnosis of a herniated disc as "highly theoretical at best." He stated:
"Upon reviewing those reports today, I don't find any specific evidence of objective findings recorded by the various examining physicians. * * *
"Assuming that the patient's description of her symptomatology is not accurate, either intentionally or unintentionally, then I find no definite evidence of injury at any time in this case, either in 1978 or 1979."
Two of claimant's examining physicians suggested that her difficulties may be rooted in underlying psychological problems. The referee concluded:
"One can come to the conclusion in this case that it is just as believable that if claimant, in fact, does have some symptoms they do not arise organically, they arise emotionally. There is no evidence that either incident gave rise to or exacerbated claimant's hysteria suggested by more than one doctor."
When the evidence presented reflects two explanations for a claimant's condition that are equally plausible to the fact-finder, and one is noncompensable, the claimant has failed to sustain the burden of proof. Gormley v. SAIF, 52 Or. App. 1055, 630 P.2d 407 (1981); Raines v. Hines Lbr. Co., 36 Or. App. 715, 719, 585 P.2d 721 (1978). We conclude that claimant's aggravation claim was properly denied.
Claimant asserts that the penalty and attorney fee award is insufficient to deter dilatory claims processing by insurers. See ORS 656.262(9); Stone v. SAIF, 57 Or. App. 808, 812, 646 P.2d 668 (1982), petition dismissed 294 Or. 442, 656 P.2d 940 (1983). The penalty and fee were awarded because Berg did not pay interim compensation within 14 days after notice of the claim. ORS 656.262(4). See Bono v. SAIF, 66 Or. App. 138, 673 P.2d 558 (1983). Berg issued its denial on the 21st day. ORS 656.262(9) provides for a penalty of up to 25 percent of the compensation awarded.
Berg argues that the penalty is sufficient, given the confusion in the law concerning the defense of untimely notice of an injury by a claimant. Notice must be *620 given within 30 days, ORS 656.262(1), or the claim is barred, unless the insurer has begun payment. ORS 656.262(3)(b). An insurer has 60 days to accept or deny a claim, ORS 656.262(6), but must begin making payments within 14 days. ORS 656.262(1). Berg argues that this places the insurer in a "Catch-22" situation.
The Board explained:
"Thus, a claimant may be barred from pursuing an untimely claim unless the insurer begins making payments on the claim. But, if insurers are required to make interim compensation payments, they will lose their right to assert the defense of lack of timely notice. This puts the insurers in the position of having to deny a claim without adequate time to make a reasonable determination of compensability or face losing the right to assert the untimely notice defense. We do not believe that the legislature intended this result.
It concluded that ORS 656.262(4) imposes a duty to pay interim compensation if the claim was not denied within 14 days, but that payment will not waive the untimely notice defense if the claim was denied within 60 days. We disagree. ORS 656.262(4) is unambiguous. If an insurer begins making interim payments, it waives its right to assert the untimely notice defense. Jones v. Emanuel Hospital, 280 Or. 147, 151, 570 P.2d 70 (1977); Logan v. Boise Cascade Corp., 5 Or. App. 636, 485 P.2d 441 (1971). That result works no injustice. Unlike issues concerning legal and medical causation, which the insurer has 60 days to investigate, it is relatively easy for an insurer to determine if notice is timely. If the defense is not raised within 14 days, it is waived.
Here Berg did not raise the untimely notice defense within 14 days. It also delayed paying interim compensation. The amount awarded as a penalty and attorney fee was within the Board's discretion. See Logan v. Boise Cascade Corp, supra, 5 Or. App. at 642, 485 P.2d 441. We find no abuse of discretion.
Affirmed.
NOTES
[1]  The referee questioned claimant's credibility. For that reason, he relied on the medical reports. We accord great weight to a factfinder's credibility evaluation. Anfilofieff v. SAIF, 52 Or. App. 127, 627 P.2d 1274 (1982). Although the referee did not directly address Dr. Weinman's medical reports, they were admitted in evidence; and the referee found that they did not support her assertions "with any degree of certainty."
